PN junctions within semiconductor devices are not infinite, terminating at the edge zones of a die. This edge effect limits the device breakdown voltage below the ideal value, V.sub.brpp, that is set by the infinite parallel plane junction. Care must be taken to ensure proper and efficient termination of the junction at the edge of the die; if the junction is poorly terminated, the device breakdown voltage can be as low as 10-20% of the ideal case. Such severe degradation in breakdown voltage can seriously compromise device design and lead to reduced current rating as well. In addition, an inefficient edge termination makes a device unstable and unreliable if the device is operated in a harsh environment or over a long period of time.
Various edge termination techniques have been developed, including, for example, field plate (FP), described in F. Conti and M. Conti, "Surface breakdown in silicon planar diodes equipped with field plate," Solid State Electronics, Vol. 15, pp 93-105, the disclosure of which is incorporated herein by reference. Another edge termination approach is field limiting rings (FLR), described in Kao and Wolley, "High voltage planar p-n junctions," Proc. IEEE, 1965, Vol. 55, pp 1409-1414, the disclosure of which is incorporated herein by reference. Further edge termination structures utilized variable lateral doping concentration (VLD), described in R. Stengl et al., "Variation of lateral doping as a field terminator for high-voltage power devices, IEEE Trans. Electron Devices, 1986, Vol. ED-33, No. 3, pp 426-428, and junction termination extension (JTE), described in V.A.K. Temple, "Junction termination extension, a new technique for increasing avalanche breakdown voltage and controlling surface electric field in p-n junction," IEEE International Electron Devices Meeting Digest, 1977 Abstract 20.4, pp 423-426, the disclosures of which are incorporated herein by reference.
The purpose of all these various techniques is to reduce electron-hole avalanche generation by lowering the peak electric field strength along the semiconductor surface and thereby shifting the avalanche breakdown location into the bulk of the device. To achieve this goal, the width of the edge termination zone (L.sub.edge) has to be several times higher than the depletion width (W.sub.pp) of the parallel-plane portion of the PN junction. For example, if L.sub.edge =2.98W.sub.pp, 98.7% of V.sub.brpp can be achieved when an "ideal edge termination," as described in Drabe and Sittig, "Theoretical investigation of plane junction termination,"Solid State Electronics, 1996, Vol. 3, No. 3, pp 323-328, the disclosure of which is incorporated herein by reference, is used. In practice, a longer L.sub.edge than the theoretical value should be used to guarantee device reliability. However, it is very important to point out that, even with very efficient edge termination, electron-hole impact generation at a rate of about 1.times.10.sup.18 pairs/cm.sup.3.s, still exists along the semiconductor surface.